The present invention relates to a data transmission system which utilizes multiplex techniques to transmit data signals among interface circuits.
In the past, a variety of multiplexing systems have been used to reduce the wiring requirements in data transmission systems. Related U.S. patent application Ser. No. 06/567,476 and U.S. Pat. No. 4,293,947 both disclose data transmission systems in which the central station is connected to a series of remote stations by a bus which transmits both clock signals and data signals. Each of the remote stations includes a counter which maintains a count of the clock pulses. Each of the remote stations has a unique address, and when the count of clock pulses equals the address, the respective remote station is enabled so as to transmit data between the remote station and the central station via the bus.
This approach to a data transmission system requires the use of a relatively sophisticated central station that receives data signals from the remote stations, processes the data signals, and then sends out commands to the remote stations. With this approach, the remote stations do not transmit data directly from one remote station to another; instead the central station always acts as an intermediary.
The data transmission system described in above-referenced application Ser. No. 921,185 allows one remote station to communicate directly with another, thereby eliminating the need for a sophisticated central station.
With both types of multiplex systems a failure in the bus can prevent reliable communication with the remote stations. In particular, the remote stations respond to the clock pulses by reading data from the bus, and a bus failure which prevents accurate transmission of data signals will cause the remote stations to read incorrect data from the bus. This can present a safety hazard when the remote stations are used to control motors, as for example in a vehicle multiplex system. Application Ser. No. 921,185 recognizes this problem and provides means for interrupting generation of the clock signals when a bus failure was detected.
It is an object of this invention to provide protection against a bus failure in a multiplex system of the type that includes a central station.